1. Field of the Invention
The present invention relates to digital image processing and, more particularly, to processing color halftone images.
2. Description of the Related Art
Monochrome halftone algorithms are carefully designed to reduce visible artifacts. Current color halftoning algorithms are usually a Cartesian product of three halftoned monochrome planes corresponding to the color components of the image. See, for example, A. Zakhor, S. Lin and F. Eskafi, "A New Class of B/W and Color Halftoning Algorithms", International Conference on Acoustics, Speech and Signal Processing, 1991.
To produce a good color halftone one has to place colored dots so that the following specifications are optimally met:
(1) The placement pattern is visually unnoticeable. PA1 (2) The local average color is the desired color. PA1 (3) The colors used reduce the notice-ability of the pattern.
The first two design criteria are easily carried over from monochrome algorithms. However, the third cannot be satisfied by a simple Cartesian product generalization of monochrome halftoning.
One of the most dominant factors in producing halftone noise artifacts is the variation in the brightness of the dots. In monochrome halftones (i.e., Black and White), this factor cannot be mitigated. In color halftoning there are however colors that could be rendered using different halftone-color sets (with different brightness variation). To be able to use those specific halftone-colors in the actual rendering the color planes would have to be correlated. Hence a simple Cartesian product generalization of monochrome halftoning will not address this problem.
In U.S. patent application Ser. No. 08/641,304, filed Apr. 30, 1996, entitled "Joint Design of Dither Matrices for a Set of Colorants" and assigned to the same entity as this application, Jan Allebach and Qian Lin describe a criterion to use colors selected to reduce the notice-ability of the pattern. In their implementation, they disable the use of certain halftone-colors when rendering some colors. However, their interpretation of the third criterion is only partial and hence their application achieves only part of the possible halftone noise reduction.
Thus, it can be seen that color halftone imaging techniques impose pattern notice-ability limits upon halftone image output devices, and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for a technique that can create better color halftone images by correctly incorporating the third design criterion (i.e., by using colors that reduce the notice-ability of the pattern).